1. Field of the Invention
The present invention relates generally to an information recording medium for recording information using, for example, a diffraction grating, and further to a method of reading the information recorded on the information recording medium and an apparatus for detecting an image recorded on the information recording medium.
2. Description of the Related Art
Conventionally, a display unit formed of a diffraction grating or a hologram not only is used for decoration or one-point design but finds wide applications as an information recording medium by being built in a part of the design of a tradable coupon for security to prevent forgery or suppress duplication.
Such an information recording medium is formed in such a manner that an optical diffraction structure formed with a pattern by the hologram or the diffraction grating or the like to prevent forgery is foiled by thermal transfer to a plastic card, a tradable coupon or a high-priced prepaid card. The reason why this type of display unit is utilized as an anti-forgery means is that the fabrication of the optical diffraction structure is technologically so sophisticated that it is difficult to duplicate the same display unit.
Jpn. Pat. Appln. KOKAI Publication No. 2003-015510, for example, discloses a display unit for displaying an image by arranging a plurality of cells as pixels, characterized in that the cells are formed of a point-type computer hologram for recording preset information as an object light, and by changing the diffraction efficiency of each cell and thus changing the brightness of the pixels on the image, a gradation image can be displayed.
Also, Jpn. Pat. Appln. KOKAI Publication No. 2003-295744 discloses an optical information recording medium having an optical information recording unit configured to form a plurality of areas including a computer hologram on the surface of a base material, wherein the computer hologram is constituted of cells as component units for recording specific information independently. In this recording medium, at least two types of computer holograms including different carrier waves are arranged in a small area and the difference of the carrier waves is larger than the frequency bandwidth of the reproduction information from the computer hologram.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2003-215319 discloses an authentication medium configured of a visual authentication unit including a diffraction structure having a hologram or a diffraction grating adapted to record an image reproducibly using visible light, and an authentication code unit formed to the size visually unrecognizable with unit images formed by the diffraction structure and arranged in a predetermined pattern thereby to make up an authentication code.
A technique for confirming whether the information recording medium is authentic with the correct information recorded is also disclosed.
In Jpn. Pat. Appln. KOKAI Publication Nos. 2003-015510 and 2003-295744 described above, cells (dots) making up “the mechanical read information” different from the pixels making up the decoration image are mixed in the hologram with each cell (dot) as a component unit, so that the authenticity is determined by taking advantage of the fact that the information unrecognizable to the naked eye appears at the time of the machine read operation.